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Thread: VNAV at Takeoff

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    VNAV at Takeoff

    Hi everybody,

    I'm runing a complete set of PM software with a FS9 B747-400. My setup uses the PMDG airfile with Posky Model/Textures. It gives me a very accurate flight model without the problems of compatibility between PM and PMDG.

    My question is about the takeoff. I make it with TO/GA mode, LNAV and VNAV armed. The first unrealistic thing happens when the VNAV mode engages at 480' AGL. The thrust jumps from TO (or D-TO) to CLB. It happens too soon. In the reality it happens at about 1500' AGL. I don't find where to tune this parameter within the CDU or the .ini files, even via the ACF files. Does somebody have an idea about this issue ?

    The second thing is about the speed management during the first segment of the climb. After the rotation, the target speed must be V2+10, that's what I set in the MCP IAS window before takeoff. But when the VNAV mode engages at 480' AGL, the target speed increases and the MCP IAS window extinguishes. It's a too low altitude for the acceleration, in the reality this acceleration height is at about 1500', until 3000' during noise abatment procedures. Again, in the reality, this "acceleration height" is available for tuning in the CDU TAKEOFF REF page. This is not implemented in the PM CDU and I don't find how to set it up. Any help will be appreciated.

    Thank you and have nice flights

    Jean-philippe

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    500+ This must be a daytime job Sean Nixon's Avatar
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    Re: VNAV at Takeoff

    Hi Jean-philippe, welcome to the forum.

    I'm not 100% up to date with flying the AFDS at the moment (and I fly 737's), but are you not engaging VNAV too soon? Shouldn't it be engaged once the aircraft is cleaned up (flaps), typically around 3000', like you said?

    Regards

    Sean

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    500+ This must be a daytime job Sean Nixon's Avatar
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    Re: VNAV at Takeoff


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    10+ Posting Member maussuam's Avatar
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    Re: VNAV at Takeoff

    Hey Jean-Philippe,

    I'm not shure if I remember right, but wasn't it the second perf init page where you can edit the thrust reduction alt ?

    Regards
    Bastian

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    Re: VNAV at Takeoff

    Some companies takeoff with VNAV armed, sone others engage it later. In all situatons the VNAV mode is built to manage the speed and thrust correctly, and PM doesn't. I'll check this second page on the PERF INIT.

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    Re: VNAV at Takeoff

    No thrust reduction field on the second page of PERF INIT. There is a T/R ALT field on LSK 5L of the THRUST LIM page. I tried to set it to 1500 but the autothrottle still engages CLB at 480' AGL. There is a long time I don't wait anymore for PM for support. I hope the comunity we are will help.

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    10+ Posting Member maussuam's Avatar
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    Re: VNAV at Takeoff

    The Boeing 737-CL Operations Manual says "Above 3000 feet AGL, engage VNAV or select normal climb speed and verify announciation."
    I remember a pilot statement, that it is his company procedure to set VNAV when reaching clean configuration and not arming it on GND.

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    Re: VNAV at Takeoff

    I have to appologize. I checked the 747-400 crew trianng manual and it says the same thing. So the pitch mode for takeoff is TO/GA until clean configuration and VNAV engagement, is that correct ?

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    500+ This must be a daytime job Sean Nixon's Avatar
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    Re: VNAV at Takeoff

    In the 737NG, the autothrottle switches from TO/GA to ARM at around 800'. You can then press N1 to initiate thrust reduction and enter CLB mode, otherwise leave it and it'll do it automatically once the TR ALT set in the FMC is reached (normal method).

    Edit: once the aircraft is cleaned up, VNAV is normally selected, perhaps around 3000'.

    Sean.

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    Re: VNAV at Takeoff

    Understood. But back to my question, where can I set the TR ALT in the FMS ? I don't see any field in the PM FMS
    and I have no answer from PM.

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